1. Field of the Invention
This invention relates to solid solutions and, more particularly, to a solid solution and process for producing a solid solution, the solid solution including at least silicon carbide and aluminum oxycarbide and, additionally, aluminum nitride.
2. The Prior Art
Structural ceramic materials which retain their strengths to temperatures on the order of 1400.degree. C. to 1500.degree. C. are desirable for their application in high temperature environments including those encountered in energy conversion devices such as gas turbines, diesel superchargers, sterling engines and the like. Currently, silicon carbide and silicon nitride are the leading contending materials for use in gas turbine engines. However, the fabrication of gas turbine engine parts from silicon carbide and silicon nitride is very expensive. One possibility for lowering fabrication costs is to sinter the parts from a preformed silicon carbide or silicon nitride powder.
These two materials should be considered as a family of ceramics because a variety of processing fabrication techniques can be applied. For example, the hot-pressed material has the highest strength while the reaction-sintered materials have the lowest strength, but are best for fabricating large or complex shapes. Hot-pressing is carried out at 1700.degree. C. to 2000.degree. C., using fine-grained powder (approximately 0.1 micrometer particle size). Additives are required to aid densification. Magnesium oxide or yttrium oxide are usually favored as the additives for silicon nitride, while aluminum oxide, boron and carbon are favored for silicon carbide.
In the process involving reaction-sintered silicon nitride, a silicon pre-form is prepared by slip casting, injection molding, or isostatic pressing. The silicon in the pre-form is thereafter converted to silicon nitride by reaction with nitrogen at 1200.degree. C. to 1400.degree. C. for several days under a nitrogen atmosphere.
Reaction-sintered silicon carbide depends upon the reaction of silicon carbide and graphite with silicon, or more simply, on the reaction of carbon with silicon. Advantageously, extremely complex shapes can be fabricated by this method. Furthermore, recent developments in the ability to sinter silicon carbide to 98% theoretical density and the anticipation that it will soon be possible to sinter silicon nitride to similar low-porosity levels is expected to lead to other new uses for these materials.
Silicon carbide has a variety of uses in many fields including, for example, additives in ferrous metallurgy, abrasives, refractories, heating elements, semiconductors and the like. The largest single use of silicon carbide on the basis of tonnage is as an additive in ferrous metallurgy. However, the most important application in terms of capitalizing on the properties of silicon carbide is its use as a refractory.
There are three major methods of preparing silicon carbide, (1) by carbothermal reduction of silica, (2) by the elemental reaction between silicon and carbon, and (3) by pyrolysis of silane compounds. However, the majority of silicon carbide is commercially manufactured in an Acheson furnace by the carbothermal reduction reaction by heating a mixture of sand (silica), coke (carbon) and sawdust. The process involves the formation of silicon carbide powder at lower temperatures (1500.degree. C. to approximately 1800.degree. C.) and subsequent recrystallization of the silicon carbide by further heating at temperatures of approximately 2000.degree. C. or above for several days. However, both aluminum oxycarbide and aluminum nitride decompose into aluminum oxide (Al.sub.2 O) and carbon and also aluminum and nitrogen, respectively, at substantially lower temperatures, approximately 1900-2100.degree. C., depending upon the particular vapor pressure. Accordingly, any aluminum or aluminum compounds present in the Acheson furnace will be volatilized and removed from the Acheson furnace along with the other gaseous by-products. This is one of the reasons that the commercially produced silicon carbide does not contain aluminum nitride or aluminum oxycarbide. Neither is silicon carbide found in solid solution with aluminum oxycarbide and aluminum nitride when these compounds are produced by other processes.
The resultant product of the Acheson furnace is an intergrown crystalline mass of silicon carbide having the alpha structure. The difference between the formation of the alpha structure silicon carbide and the beta structure is probably controlled by the impurities or materials in solid solution with the silicon carbide rather than by the temperature. For example, one researcher has noted that aluminum additions in a nitrogen overpressure beyond 1% can result in an extensive formation of alpha structure silicon carbide in the solid solution. However, the problem of growing crystals of silicon carbide is a very difficult problem since the temperatures involved would be at the melting point (if under an overpressure) and, therefore, the results obtained by the researchers are only generally suggested results for someone willing to grow crystals of silicon carbide under pressure. Other experimental work has also been performed by other researchers which was conducted at fairly low temperatures in the order of 1500.degree. C. to 2000.degree. C. by chemical vapor deposition techniques.
Previous experience with the formation of silicon carbide by the carbothermal reduction of silica has been rather extensive and has readily confirmed that (1) silicon carbide is formed by a gaseous phase reaction between carbon and silica, (2) that the quantity of available surface area of both specie (carbon and silica) is important and (3) that the reaction is catalyzed by iron. Importantly, however, we have also determined through our experiments, that when working with very high purity chemicals, particularly in the absence of iron or other transition metals, there is evidence of alpha silicon carbide being formed. On the other hand, when the reactants are doped with even a small amount of iron (0.1% up to 1%, by weight) beta silicon carbide is easily formed.
Aluminum nitride has a density and a coefficient of thermal expansion very near to that of silicon carbide as well as having a very high melting point. Upon crystallization, aluminum nitride forms into the wurtzite or hexagonal structure. Additionally, like silicon carbide, aluminum nitride does not melt easily unless there is an overpressure above about 1 atmosphere and decomposes to aluminum and nitrogen at about 2000.degree. C.
Previous studies of the aluminum oxide/aluminum carbide system have noted that there is one specie of oxycarbide that is similar in structure to aluminum nitride. This is the Al.sub.2 OC oxycarbide which as the same wurtzite structure as aluminum nitride. This aluminum oxycarbide has also been found to form a complete solid solution with aluminum nitride. The present studies which resulted in this invention were concerned about the ability to form this particular oxycarbide in the absence of a small amount of nitrogen. The experiments of the present invention have resulted in the formation of the Al.sub.2 OC oxycarbide from aluminum oxide by carbon reduction in the absence of nitrogen.
A recent discovery has been made of a new group of materials called the SiAlONs. The term "SiAlON" is an acronym derived from the chemical symbols of the constituents silicon, aluminum, oxygen and nitrogen. For example, see my U.S. Pat. No. 3,960,581 issued June 1, 1976 for PROCESS FOR PRODUCING A SOLID SOLUTION OF ALUMINUM OXIDE IN SILICON NITRIDE. The SiAlONs are, however, only a small part of a vast family of silicon metal oxynitrides that are isomorphous with known minerals. Advantageously, the SiAlONs offer a greater flexibility in processing than do silicon nitride and silicon carbide, but because they are new materials, they have received comparatively little attention.
In recognition of the interest in high-strength ceramic materials having the desired physical properties, it would be an advantage in the art to provide a ceramic material consisting of a solid solution of silicon carbide, aluminum nitride and aluminum oxycarbide. Such a solid solution and process for producing the same is disclosed and claimed herein.